The testing of oil and gas wells is of great importance for determining reservoir properties and production capacity of a hydrocarbon containing reservoir. Such testing is preferably made with a drill string, during so-called drill string testing (DST), during which a zone of interest is isolated by temporary packers, so that fluid from the reservoir zone may flow into the space between said packers.
U.S. Pat. No. 5,799,733 discloses a down-hole tool for early evaluation of a reservoir, primarily for taking samples of open-hole reservoir fluid. In said publication is described inflatable packer elements for isolating an open-hole reservoir interval of interest, a down-hole pump driven electrically or by a mud motor and providing a mud return to a drill string/test string or the annulus above the packers, and further are described a sample chamber and sensors for the measurement of fluid properties. Technology enabling an extended testing like the continuous mixing of mud and reservoir fluid during controlled conditions is however not disclosed, but several places give warnings against the risk for loss of pressure control, see for example column 16, lines 33-42 in U.S. Pat. No. 5,799,733. For embodiments having an electrically driven pump the formation fluid is fed to a well bore test string in order to eliminate the risk for loss of pressure control. For embodiments having a mud pump it is not possible to feed formation fluid into the upper part of a drill or test string, and for all such embodiments severe warnings are expressed against the risk of losing pressure control.
There is a demand for an assembly and a method for transient and continuous testing of an open portion of a well bore, without the above-mentioned limitations.